Why do my two identical VM's have such diff RAM usage?

Why is my RAM usage so different between the MASTER (NS1) and SLAVE (NS2).

Is this normal? Both exist on two different VPS nodes and both are in separate cities. Same hosting provider.

Both running Ubuntu 12.04 LTS + MySQL server + PDNS (authoritive)

NS2 = SLAVE NS1 = MASTER

root@ns1:~# free -m
             total       used       free     shared    buffers     cached
Mem:           768        364        403          0          0         28
-/+ buffers/cache:        336        431
Swap:            0          0          0

root@ns2:~# free -m
             total       used       free     shared    buffers     cached
Mem:           768         39        728          0          0         12
-/+ buffers/cache:         26        741
Swap:            0          0          0